Temperature-Controlled Reaction–Separation for Conversion of CO2 to Carbonates with Functional Ionic Liquids Catalyst

Carboxylic acid-based ionic liquids (CAILs) were designed and synthesized. They displayed temperature-dependent dissolution–precipitation transitions in propylene carbonate (PC) by controlling the chain length of the carboxyl group. It was found that it could be attributed to the temperature-controlled hydrogen-bond formation/broken between the CAILs components and PC by NMR investigations and DFT calculations. Such a unique phase behavior was successfully utilized for the cycloaddition reaction of CO2 with epoxides. Due to the activation of epoxide assisted by the hydrogen bond, the optimal ILs could show a 92% yield of PC within 20 min and quickly precipitate out from a homogeneous system at room temperature for easy recycling. The reversible phase transition phenomenon supplied an efficient way to combine activity and recovery of homogeneous catalysts, which is a benefit for energy-saving and industrial applications.